SelectMany

string[] presidents = {
"Adams", "Arthur", "Buchanan", "Bush", "Carter", "Cleveland",
"Clinton", "Coolidge", "Eisenhower", "Fillmore", "Ford", "Garfield",
"Grant", "Harding", "Harrison", "Hayes", "Hoover", "Jackson",
"Jefferson", "Johnson", "Kennedy", "Lincoln", "Madison", "McKinley",
"Monroe", "Nixon", "Pierce", "Polk", "Reagan", "Roosevelt", "Taft",
"Taylor", "Truman", "Tyler", "Van Buren", "Washington", "Wilson"};

IEnumerable<char> chars = presidents.SelectMany(p => p.ToArray());

Employee[] employees = Employee.GetEmployeesArray();
EmployeeOptionEntry[] empOptions = EmployeeOptionEntry.GetEmployeeOptionEntries();

var employeeOptions = employees
                      .SelectMany(e => empOptions
                                       .Where(eo => eo.id == e.id)
                                       .Select(eo => new {
                                                           id = eo.id,
                                               optionsCount = eo.optionsCount }));

foreach (var item in employeeOptions)
   Console.WriteLine(item);